As solid-state imaging devices (image sensors) using photoelectric conversion elements which detect light to generate electric charges, CMOS (complementary metal oxide semiconductor) image sensors have been put into practical use. CMOS image sensors are being widely applied as parts of digital cameras, video cameras, monitoring cameras, medical endoscopes, personal computers (PC), mobile phones, and other portable terminal devices (mobile devices) and various other types of electronic apparatuses.
A CMOS image sensor has a floating diffusion (FD) amplifier having, for each pixel, a photodiode (photoelectric conversion element) and floating diffusion layer. For readout, the mainstream type is the column parallel output type that performs selects a certain row in a pixel array and simultaneously reads the pixels out to a column output direction.
Each of the pixels in a CMOS image sensor basically includes, for example, one photodiode and, with respect to that, four elements of a transfer gate comprised of a transfer transistor, a reset gate comprised of a reset transistor, a source follower gate (amplification gate) comprised of a source follower transistor, and a selection gate comprised of a selection transistor as active elements. Further, each pixel may be provided with an overflow gate (overflow transistor) for discharging an overflow charge overflowing from the photodiode in an accumulation (storage) period of the photodiode.
The transfer transistor is held in a non-conductive state in the charge accumulation period of the photodiode and is held in a conductive state by application of a driving signal to the gate in a transfer period transferring the accumulated (stored) charge in the photodiode to the floating diffusion FD and transfers the charge which is photoelectrically converted in the photodiode to the floating diffusion FD.
The reset transistor resets the potential of the floating diffusion FD to the potential of a power supply line by a reset signal being given to its gate.
The floating diffusion FD is connected to the gate of the source follower transistor. The source follower transistor is connected through the selection transistor to a vertical signal line and configures a source follower together with a constant current source of a load circuit outside of the pixel portion. Further, a control signal (address signal or select signal) is given to the gate of the selection transistor whereby the selection transistor turns ON. If the selection transistor becomes ON, the source follower transistor amplifies the potential of the floating diffusion FD and outputs a voltage in accordance with that potential to the vertical signal line. Through the vertical signal line, voltages output from the pixels are output to a pixel signal readout circuit comprised of a column parallel processing part. In the column-parallel processing, the image data is for example converted from an analog signal to a digital signal and is transferred to a later stage signal processing part where it is subjected to predetermined image signal processing whereby a desired image is obtained.